This invention relates to an improvement in processes for dewaxing oils and deoiling the slack wax by-product produced from said dewaxing. More specifically, the invention relates to those solvent dewaxing-solvent deoiling processes wherein a toluene-MEK solvent used to dewax the waxy oil and deoil the slack wax by-product must be maintained in an essentially anhydrous condition.
In the art of petroleum refining, petroleum oil feedstocks are conventionally dewaxed by solvent dewaxing. In one such process, a solvent, e.g. a blend of 50% toluene and 50% MEK, is admixed with the waxy feedstock, and the resulting solvent-feedstock mixture is then passed through a series of conventional, double-piped, scraped-surface chillers. As the solvent-feedstock blend is passed through the chillers, wax gradually crystallizes therefrom, and a slurry is recovered from the final chiller that consists of solvent, oil, and crystallized slack wax. After the slack wax is separated from the slurry by filtration, a dewaxed oil-solvent filtrate remains.
In order to convert the slack wax by-product into the marketable products of foots oil (i.e., a mixture of oil and soft wax) and a fully refined petroleum wax (i.e., wax containing less than 0.5 weight percent oil), the slack wax must be deoiled, usually by the solvent deoiling process. In this process, which is similar to solvent dewaxing, the slack wax is heated above its melting temperature (usually between about 120.degree.-180.degree. F.) and is then combined with solvent, which usually is of the same solvent composition as that used in solvent dewaxing. The resulting blend is passed through a second series of double-piped, scraped-surface chillers, wherein cooling is effected at a rate of between about 1.degree. and 10.degree. F./min. During chilling, wax crystals deposit on the walls of the inner pipes of the chillers while the rotating scrapers within the chillers gently remove the deposited wax from the inner surface of the inner pipes, thereby producing a slurry containing wax crystals. This slurry gradually increases in wax crystal content as it is passed from one chiller to the next, and, ultimately, a product slurry at a temperature between about 20.degree. and 85.degree. F. is recovered from the last chiller. This product slurry is then filtered to produce a foots-oil solvent filtrate and a filter cake containing solvent and fully refined petroleum wax.
Solvent utilized in the solvent dewaxing-solvent deoiling process as just described is recovered from the dewaxed oil-solvent filtrate, the foots oil-solvent filtrate, and the fully refined petroleum wax-solvent filter cake. This is usually accomplished by a combination of flash evaporation and steam stripping operations, which produce a fully refined petroleum wax, dewaxed oil, foots oil, and various streams of dry solvent containing less than about 0.5% water and wet solvent containing about 1 to about 6% water. Dry solvent is normally recycled directly to facilities for storing solvent required in the solvent dewaxing-solvent deoiling operations hereinbefore described. Meanwhile, the wet solvent, or at least some streams thereof, are at least partially dehydrated before being recycled to such storage facilities.
When the solvent utilized in the solvent dewaxing-solvent deoiling chillers comprises at least 50% toluene with the balance being MEK, the wet solvent streams obtained from steam stripping operations will consist of a two-phase liquid containing proportionally more toluene than that contained in the solvent storage facilities. For example, when the solvent fed from storage to the dewaxing and deoiling chillers contains a 50-50 mixture of toluene and MEK, the wet solvent streams will often contain 70% toluene (on a water-free basis). To partially dehydrate some or all of such wet solvent streams, a decanter is used to separate the wet solvent into a water-lean phase and a water-rich but toluene-free phase. The water-lean phase is then recycled to the solvent storage facilities and therein combined with the dry solvent. The water-rich phase, on the other hand, is usually subjected to azeotropic distillation to reduce its water concentration, thereby leaving an azeotropic vaporous fraction from which a small amount of MEK may be recovered by extraction in a hydrocarbon stream, such as a stream of melted slack wax about to be deoiled. The MEK so recovered is then recycled as a portion of the solvent used in the solvent deoiling chillers.
One problem in the dewaxing-deoiling process as just described lies in combining the water-lean solvent phase removed from the decanter with the dry solvent. Because this phase usually contains about 1% water, its introduction into the dry solvent, which usually contains less than about 0.5% water, necessarily results in increased icing in the dewaxing chillers, some of which operate at temperatures as low as -40.degree. F. More importantly, increasing the water content in the solvent used in the dewaxing-deoiling operations throws an increased load on the chillers and evaporators. It is estimated that, for every one percent increase in water content in the solvent, a three percent increase in heat load is thrown on the chillers and flash evaporators.
It is therefore an object of the invention to provide a process for dehydrating those wet solvents from which an essentially one-phase liquid containing more than about 60% toluene, with the balance being MEK and water, may be recovered in a conventional liquids separator, such as a decanter.
It is a specific object of the invention to dehydrate the water-lean phase of a wet solvent stream obtained in solvent dewaxing-solvent deoiling processes utilizing a blend of toluene and MEK, said water-lean phase being essentially one-phase and containing at least 60% toluene with the balance consisting of water and MEK.